There are presently five basic systems used in large scale pressure irrigation. Selection of any one depends on the particular application requirements. Each approach incorporates its own characteristic advantages and disadvantages.
Probably the first large scale pressure irrigation technique involved the use of "hand lines". Hand lines are basically a series of water conduit sections capable of being coupled, uncoupled and hand moved from one setting to another. A common example of hand lines might be a 1,300 foot linear series of four inch conduit coupled in thirty foot lengths with riser pipes every sixty feet. Sprinklers attached to the riser pipes emit a spray of water over a radius of approximately forty feet. After a normal setting of eight to twelve hours, the entire series is hand moved approximately sixty feet and reset parallel to the previous conduit set.
Hand lines require a very large labor cost and suffer the associated inefficiencies. They also suffer from water and power waste created by sprinkler overlap and an inherent inability to interrelate with changing weather conditions due to a minimum inevitable ten or eleven day lapse between waterings. In addition, the lapse creates a continual fear of unexpected hot weather which causes constant overwatering.
Subsequent development of more automated systems has resulted in less frequent use of hand lines. However, for lands with very irregular shapes or extremely hilly contours, hand lines are often still used.
A subsequent development to hand lines has been the "wheel line" system. Wheel line systems involve sections of water conduit coupled together and acting as an axis for appropriately spaced powered wheels. The line is rolled to the next setting position after the system is drained and is reconnected to a main line water source.
Basically, a wheel line is a rolling hand line. It eliminates part of the labor expense relating to uncoupling, moving, and recoupling the hand line sections but retains much of the hand line inefficiencies, including water and power waste created by the necessary sprinkler overlaps, inflexible watering cycles and constant overwatering.
While capable of irrigating slightly hilly lands, wheel lines lose the ability of hand lines to irrigate hilly lands and to accommodate irregularly shaped land parcels.
A typical wheel line irrigates at least thirty-five acres, thus allowing large irregular fields to be irrigated by several wheel lines. However, the shape of the area irrigated is substantially limited to rectangular configurations.
Another more recent development in irrigation is the "lateral move" system. Lateral move systems consist basically of conduit sections trussed and mounted atop appropriately spaced wheeled towers. The towers and trusses along with much electronic and mechanical hardware, allow lateral move systems to continuously move across a field while full of water and in operation.
At present there are three methods commercially used for connecting lateral moves to a main water supply. One method uses a very large pump system usually located in the center or at one end of the conduit span designed to pump from a trough or ditch. Accordingly, this method is useful only for flat land.
The second method uses a hose connected to a pressurized main line. As the system moves, the hose drags along the ground until all slack is gone. It then becomes necessary to disconnect and move the hose to the next main line valve. The hose creates additional water resistance and thus requires higher operating pressure. However, this method of connection to the water source represents a relatively simple, inexpensive mechanical connecting apparatus, so initial start-up costs are minimized. The hose hookup system thus offers a relatively cost-effective method for smaller scale applications. Hose hookups allow use of the lateral move on slightly hilly land.
A third style of hookup for lateral moves is comparatively complex. Electronic and mechanical hardware allow automatic hookup and unhooking to a pressurized main line while the sprinkler line moves constantly forward with no shutdown of water flow. The approach eliminates manual hookups. It also allows the system to accommodate slightly hilly contours and alleviates many of the problems associated with pumping from a trough. However, initial equipment costs and malfunction potential are the highest of the three methods.
Lateral move systems represent an advance in technology. Forward sprinkler motion reduces the sprinkler overlap inefficiency, water cycle inflexibilities, and labor costs of the wheel line systems. Before the advent of the present invention, lateral move systems have been perhaps among the most efficient systems available. The overall cost for such a system, however, limits the effective use only to large scale farming. Typically, one lateral move will irrigate one or two square 320 acre sections. Lateral moves with hose hookups effectively accommodate somewhat smaller sections with increased inefficiencies.
Another significant advancement has developed in "center pivot" irrigators. Basically, a center pivot is a lateral move attached at one end to a stationary pivot. This causes the unit to revolve about the pivot, which also functions as a connection to the water supply. This eliminates nearly all of the lateral move water hookup technology and thus results in a simpler, less expensive machine. However, with the change in travel direction from linear to rotational, comes a completely different set of difficulties.
In relation to lateral moves, spray pattern efficiencies of center pivots are hampered and pressure losses are increased because of the need to linearly increase water output in proportion to the sprinkler distance from the pivot. The linear change in output also produces very high instantaneous application rates approaching the outer end of the water conduit. This can create runoff problems for long conduit spans in tight soil situations.
Center pivots must also employ some systems to apply water at corners of a section missed by its circular orbit. This additional and usually complex equipment adds considerably to the original equipment cost and complexity. The cost generally increases with the more corner land accommodated. Corner accommodation systems for center pivots represent comparatively larger additional water and power wastes. Disregarding corner systems, the cost per acre for center pivot irrigation is a function of the radius covered. The larger the span length (radius), the lower the cost per acre. A center pivot may effectively accommodate a 160 acre section. For larger sections, the cost per acre decreases but the instantaneous application rate increases.
For supplemental irrigation applications, some systems include rotatable wheel carriages. The wheel carriage may be rotated 90.degree. to facilitate tractor transport. Thus, a single line can be transported from one center connection to another. This approach is limited to large acreage multiples, incurs additional buried main line expense, and suffers from labor inefficiencies. Also, with this approach, it becomes very difficult to use a corner system. Thus, between 15% to 23% of the land remains unwatered.
The fifth of the five forms of large scale pressure irrigation systems is the "reel" irrigator. A reel irrigation system incorporates a flexible hose combined with a hose reel. One hose end is connected to the water source and the other to a reel on a small cart carrying a high volume "big gun" type sprinkler. Alternatively, the reel is held stationary and only the sprinkler is mounted to the cart. A tractor tows the cart to the end of the field or until the reel is empty. A motor on the hose reel then slowly reels in the hose and cart as the sprinkler is operated. The cart may be pivoted about the water source and then towed again out to the end of the hose length. The cart motor can then be reactivated and the sprinkler operated as the cart moves back to the water supply connection. The tractor is then used to tow the reel to another spot for subsequent hookup and operation.
Pressure losses from the extreme length of hose and the necessary hose diameter along with the operating pressure necessary for high volume big gun sprinklers result in very high operating pressures and consequent high energy usage. Sprinkler travel geometry of a reel irrigator creates an inevitable large fluctuation in application amounts resulting in large water and power waste.
Reel irrigators, however, enjoy a degree of success on hilly and irregular shaped lands. It is in these areas where they outperform the other four irrigation systems and are often used. Otherwise, the high operating pressures, large water and power wastes, along with labor cost and associated inefficiencies render them cost ineffective.
The reel pays out the hose onto the ground surface as the cart is moved to the limit of the hose length. Thus, to avoid the hose dragging over and damaging the crop, the cart must be kept in line with the hose. This severely limits use of the otherwise independently movable cart to a "yo yo" motion.
A need remains for an irrigation system that effectively combines advantages of several individual systems listed above while minimizing the disadvantages.